Speciation Studies of Metals in Trace Concentrations: The Mononuclear Uranyl(VI) Hydroxo Complexes

摘要

A direct luminescence spectroscopic experimental setup for the determination of complex stability constants of mononuclear uranyl(VI) hydrolysis species is presented. The occurrence of polynuclear species is prevented by using a low uranyl(VI) concentration of 10(-8) M (2.4 ppb). Time-resolved laser-induced fluorescence spectra were recorded in the pH range from 3 to 10.5. Deconvolution with parallel factor analysis (PARAFAC) resulted in three hydrolysis complexes. A tentative assignment was based on thermodynamic calculations: UO22+, UO2(OH)(+), UO2(OH)2, UO2(OH)(3)(-). An implementation of a Newton-Raphson algorithm into PARAFAC allowed a direct extraction of complex stability constants during deconvolution yielding log(beta(1M,1C))(1:1) = -4.6, log(beta(1M,1C))(1:2) = -12.2, log(beta(1M,1C))(1:3) = - 22.3. Extrapolation to standard conditions gave log(beta(0))(1:1) = -3.9, log(beta(0))(1:2) = -10.9, and log(beta(0))(1:3) = -20.7. Luminescence characteristics (band position, lifetime) of the individual mononuclear hydroxo species were derived to serve as a reference data set for further investigations. A correlation of luminescence spectroscopic features with Raman frequencies was demonstrated for the mononuclear uranyl(VI) hydroxo complexes for the first time. Thereby a signal-to-structure correlation was achieved and the complex assignment validated.